Communications systems having a plurality of portable communication units (PCU) and at least two fixed communication units (FCU) encounter difficulties in obtaining or selecting the optimum (or best) communication links. For example, in a standard Cordless Telephone (CT2) system, when a portable communication unit (PCU) requests a communication link, the first FCU to detect the request will grant the communication link for communicating with the requesting PCU. Unfortunately, due to the random timing of the channel scanning by the FCUs, the first FCU to detect the request may not be the closest to the requesting PCU. That is, it is not the "best" or optimum FCU to grant the link for facilitating communication with the requesting PCU.
Several methods have been proposed for selecting the "best" FCU in a CT2 system. Unfortunately, all the proposed methods have some drawbacks. A first suggested method, upon a request for a communication link by a PCU, is to allow the FCU to grant a communication link immediately only when the absolute received signal strength is above a threshold value. With this method, if no received signal strength is above the threshold value after the passage of a fixed time limit, the FCU will lower its threshold value for allowing the granting of the communication link if the PCU is still requesting a communication link. However, with this method, the FCU will answer the communication link request if it received the communication link request above the threshold value and will automatically grant the communication link. Therefore, the first method does not ensure the selection of the best FCU, it merely increases the probability of best FCU selection. On the other hand, if at least two FCUs are scanning the channels and the at least two FCUs are receiving the communication link requests at signal strengths above the threshold value, and if the FCU having the weakest signal receives the communication link request first, it is more probable that the FCU having the weakest signal strength will grant the communication link.
A second proposed method was always to require a high threshold signal strength before granting the link, however, this technique prevents a distant FCU from granting the communication link when another FCU is nearer. Unfortunately, this method also prevents a distant FCU from granting the link even if the distant FCU is the only base receiving the communication request. This technique accomplishes strong FCU selection at the expense of reduced system range.
A third proposed method was to reduce the FCU output power during the transmission of the communication link grant message. The theory is that if the PCU is unable to hear the weak link grant, the PCU will continue to request a link until a more proximate FCU (if any) scans the channel, hears the request, and grants the link. Like the second method, this method works fairly well if there is a FCU sufficiently proximate to the PCU. Unfortunately, it has the same drawback of preventing a farther FCU from granting the communication link even if the farther FCU is the only FCU, thus again accomplishing strong FCU selection at the expense of reduced system range. This drawback can be overcome by designing the FCU so that it "remembers" the first communication link request. Then when the FCU receives a second link request thereafter from the same portable on a different channel, the FCU will interpret that to mean that no other FCU was able to grant the communication link. The FCU then returns to a full-power to grant the communication link. Like the first method, this two-level approach does not ensure the selection of the best FCU, but merely increases the probability of selecting the best FCU. Due to lack of synchronous channel scanning, the timing problems affecting below-threshold FCU in the first method still applies. For example, consider when two FCUs are in range of the PCU with one much closer than the other, but both sufficiently distant to cause the PCU to receive their response at the low power level. In this case, the FCU that scans the channel first will be the FCU that grants the communication link at the higher power level irrespective of whether that FCU is best suited among the two FCUs.
Thus what is needed is a method for automatically selecting the FCU that is best suited for granting the communication link to the requesting PCU.